Even in communications applications, optical power propagated by optical fiber is tending to increase, due to the extension of non-repeater distance and the increase in multiplex numbers due to wavelength division multiplexing (WDM) technology, and it is considered possible that the average power may reach several watts sometimes or later. Attempts are also being made to transmit power by optical fiber.
When high power light is propagated in an optical fiber, it is known that there occurs an optical fiber destruction phenomenon called a fiber fuse, which is due to fine dirt or the like on the light transmission path or the optical connector. Specifically the dirty portion for example ignites, fusing the core of the optical fiber, and the fusing phenomenon progresses towards the light source, with the result that the optical fiber is destroyed as the periodic formation of micron-sized voids takes place in the core of the optical fiber, as shown in FIG. 6. The propagation velocity of this fiber fuse phenomenon is from around 100 cm to several meters a second, depending on the type of optical fiber and the light intensity.
Destruction caused by a fiber fuse progresses as long as the optical power density in the core does not go to or below a certain value. Since the optical power is higher closer to the light source, if left, the fiber fuse phenomenon will ultimately reach the light source, damaging the light source.
As means for preventing the progress of the fiber fuse phenomenon, there has been proposed the midway insertion of an optical fiber having an enlarged core diameter (see Patent Reference 1 (U.S. Pat. No. 6,640,043)). The sudden change in the mode field diameter is controlled by providing each end of the core portion having the enlarged diameter with a tapered portion that mitigates the change in core diameter.